Update: Ignore, I completely misread the OP's requirements

my $count = 0; 
for ( @array ) {
  $count += s/PERL/Perl/g;
}
print $count;
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Update 2: This seems more like what you want:

my @numbers; 
for ( 0 .. $#array ) {
  push @numbers, $_ if $array[$_] =~ s/PERL/Perl/g;
}
print join ', ', @numbers;
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Update 3: ...which, I see now, actually adds very little value to johngg's post above :-)